Casein Kinase I (“CKI” or “CK1”) was one of the first serine/threonine protein kinases to be isolated and characterized (Gross, S. D. and R. A. Anderson. 1998. Cell Signal 10:699–671). It is a ubiquitous enzyme that can be found in the nucleus and the cytosol of cells and bound to the cytoskeleton or to cell membranes. The CKI (CK1) family is a family of multiple isoforms encoded by at least seven distinct genes (CKIα, β, γi, γ2, γ3, δ, E; Desdouits, F. et al. 1995. J. Biol. Chem. 270:8772–8778). These isoforms exhibit more than 50% amino acid identity within the catalytic domains, but contain divergent carboxy-termini (Graves, P. R. and P. J. Roach. 1995. J. Biol. Chem. 270:21689–21694; Cegielska, A. et al. 1998. J. Biol. Chem. 273:1357–1364; Gietzen, K. F. and D. M. Virshup. 1999. J. Biol. Chem. 274:32063–32070). It has been reported that at least two CKI isoforms, CKIδ and CKIε, are regulated by auto-phosphorylation at their COOH-termini. These studies suggest that the phosphorylated COOH-terminus may act as a pseudo-substrate that inhibits enzyme activity. Elimination of autophosphorylation by either truncation of the COOH-terminus or by dephosphorylation has been found to result in increased kinase activity (Graves, P. R. and P. J. Roach. 1995. J. Biol. Chem. 270:21689–21694; Cegielska, A. et al. 1998. J. Biol. Chem. 273:1357–1364).
In the central nervous system (CNS), CKI appears to play a role in regulation of circadian rhythm (Camacho, F. et al. 2001. FEBS Lett. 489:159–165) and intracellular trafficking (Murakami, A. et al. 1999. J. Biol. Chem. 274:3804–3810; Panek, H. R. et al. 1997. EMBO J. 16:4194–4204; Wang, X. et al. 1996. Mol. Cell. Biol. 16:5375–5385). In the neostriatum, CKI has been found to phosphorylate DARPP-32 (Dopamine and cAMP-Regulated Phosphoprotein, molecular mass 32 kDa; also known as “DARPP-32”) at Ser137 (Desdouits, F. et al. 1995. J. Biol. Chem. 270:8772–8778). Phosphorylation at Thr34 by cAMP-dependent protein kinase (PKA) converts DARPP-32 into an inhibitor of protein phosphatase-1 (PP1), a process that is critical for the actions of dopamine in the neostriatum (Greengard, P. et al. 1998. Brain Res. Rev. 26:274–284; Greengard, P. et al. 1999. Neuron 23:435–437). Phospho-Thr34 is dephosphorylated by PP2B (also named calcineurin or Ca2+/calmodulin-dependent phosphatase) and the phosphorylation of Ser137 influences the ability of phospho-Thr34 to be dephosphorylated by PP2B (Desdouits, F. et al. 1995. Proc. Natl. Acad. Sci. USA 92:2682–2685; Desdouits, F. et al. 1998. Biochem. J. 330:211–216). Cyclin-dependent kinase 5 (“cdk5,” “Cdk5” or “CDK5”) was originally identified as a homologue of p34cdc2 protein kinase. Subsequent studies have shown that unlike Cdc2, Cdk5 kinase activity is not detected in dividing cells. Instead, the active form of Cdk5 is present only in differentiated neurons, where it associates with a neuron-specific 35 kDa regulatory subunit, termed p35. Cdk5/p35 plays a variety of roles in the developing and adult nervous system.
Studies of mice in which the gene encoding either Cdk5 or p35 has been disrupted have indicated that both mutants exhibit abnormalities in the laminar structure of the cerebral cortex (Chae, T. et al. 1997. Neuron 18:29–42; Ohshima, T. et al. 1996. Proc. Natl. Acad. Sci. USA 93:11173–11178). Cdk5/p35 is localized at the leading edge of axonal growth cones, and inactivation of Cdk5 in cultured neurons inhibits neurite outgrowth. Recent studies have linked mis-regulation of Cdk5 to Alzheimer's disease (Kusakawa, G. et al. 2000. J. Biol. Chem. 275:17166–17172; Lee, M. S. et al. 2000. Nature 405:360–364; Nath, R. et al. 2000. Biochem. Biophys. Res. Commun. 274:16–21; Patrick, G. N. et al. 1999. Nature 402:615–622). In these studies, conversion of p35 to p25 by the action of calpain causes prolonged activation and altered localization of Cdk5. In turn, Cdk5/p25 can hyperphosphorylate tau, disrupt cytoskeletal structure and promote apoptosis of primary neurons. Recent studies have shown that Cdk5 also phosphorylates DARPP-32 at Thr75 (Nishi, A. et al. 2000. Proc. Natl. Acad. Sci. USA 97:12840–12845; Bibb, J. A. et al. 1999. Nature 402:669–671). Phosphorylation of DARPP-32 at Thr75 by Cdk5 influences phosphorylation of Thr34 in DARPP-32 by PKA and plays an important modulatory role in the DARPP-32/PP1 cascade (Bibb, J. A. et al. 1999. Nature 402:669–671). Finally, recent studies have suggested that glutamate antagonists are anti-carcinogenic, an effect that was calcium dependent (Cavalheiro, E. A. et al. 2001. Proc. Natl. Acad. Sci. USA 98:5947–5948; Rzeski, W. et al. 2001. Proc. Natl. Acad. Sci. USA 98:6372–6377).
Hence, despite the knowledge about the actions of CKI and Cdk5 in the CNS, as discussed hereinabove, surprisingly little is known about the regulation of these two kinases by first messengers. Therefore, there is a need in the art to provide new methods of screening that can be used to develop novel compositions or drugs that can be used to treat diseases or disorders related to the regulation of CKI and Cdk5. Furthermore, there is a need to develop treatments for such diseases or disorders that are due, at least in part, to an aberration or dysregulation of an intracellular signaling pathway regulated by CKI and/or Cdk5. The present invention provides such methods and compositions.
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.